Karl-birger persson



MI'CI 17, 1964 `KARL-BIRGER PERssoN 3,125,698

CERAMIC To METAL SEAL y Filed June s, 1953 @w i F v INVENTOR. KHHL-BIREEFL PERS: UN

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United States Patent 3,125,698 CERAMIC T METAL SEAL Karl-Birger Persson, Boston, Mass., assigner to Radio Corporation of America, a corporation ofDelaWare Filed June 8, 1953, Ser. No. 360,193 36 Claims. (Cl. 313-220) This invention relates to ceramic to metal seals and more particularly to such seals which are especially suitable for high frequency electron tubes capable of handling relatively large amounts of'power.

It is a principal object of my invention to provide a bond or seal between ceramic and metal members which may be fabricated with ease and having improved resistance to thermal shock;

Another object is the provision of such a composite seal through which'radio frequency currents of substantial power and high frequency may be passed.

A further object is the provision of an electron tube having an improved high frequency high power current lead-in and seal.

In accordance with my invention, I effect a flexible joint between a ceramic member'an'd a relative massive metallic member by means of a thin walled flexible metallic member. The thin' walled'member is butt-sealed to the end of the ceramic member. The thin walled member is also sealed to the metallic member at some distance from the seal formed with the ceramic member. The component parts of the seal are so arranged that the thin walled member is supported, and hence, does not bear any substantial mechanical stresses and is extremely thin. As a consequence, stresses which, as for example, occur during temperature cycling of the parts'and would otherwise damage or destroy the seal are dissipated in the ductile thin walled member.

My invention, as well as further. advantages and objects thereof, will be best understood from the following description and drawing in which:

FIG. 1 is a perspective view partially in section for convenience of a metal to ceramic seal constructed in accordance with my invention;

FlG. 2 is a sectional view thereofi'on an enlarged scale;

FIG. 3 is a sectional view of a portion of an electron tube utilizing such a seal;

FTG. 4 is a sectional view of another seal constructed in accordance with my invention.

Referring now to the drawings in detail and particularly to FiG. 1, metallic member is shown sealed to annular ceramic body or member 11 through annular thin-walled metallic member 12 U-shaped in cross section. The peripheries' of thin walled member 12 are sealed to metallic member 10 at 13 and 14. Thin-walled member 12 is also sealed to the end 11 of ceramic body 11 as shown at 15 in a region spaced from the seals at 13 and 14. Ceramic body 11 may be tubular or cylindrical, and the end 11 thereof may be readily ground smooth and made planar. lf desired, the end surface 11' may be formed and polished to a high degree of precision and smoothness. However, as will be made evident, this is not necessary. While the end wall 11 may thus readily be made smooth and planar, this is not the case when the same is attempted with respect to the curved surfaces of ceramic body 11. While one surface of thin walled member 12 is sealed to ceramic member 11 as indicated at 15 the opposite surface is supported by a tubular spacer or backing member 21 which has substantially the same diameter and wall thickness as ceramic member 11 and as will be seen may also be of the same material.

I preferably form such seals as follows: After bringing the end surfaces 11 of ceramic body 11 to a substantially smooth and planar state l coat the same with a suspension of titanium hydride in acetone as by means of a camels ice hair brush or other suitable means such as spraying. The titanium hydride layer is shown at 19 in FIG. 2. A sheet Zt of brazing material preferably of pure silver about two mils thick and trimmed to be substantially coextensive with the end of ceramic body 11 is placed intermediate ceramic body 11 and thin walled member 12.

Thin walled member 12 must be extremely thin. The thickness of member 12 is critical since it must be so thin that stresses which would otherwise be generated between the ceramic and thin walled members during fabrication and use are dissipated in the thin walled member which is constrained or stretched as the case may be by the ceramic material bonded thereto.

l have found that the use of a thin walled member 12 of nickel having a thickness of less than .O10 of an inch and ranging from about .005 to .007 of an inch in thickness permits the fabrication of successful seals. The thin walled members are readily formed of sheet nickel of such thickness by stamping or as desired. The seal between thin walled member 12 and ceramic body 11 is best made with the parts under pressure and at a temperature somewhat above the melting point of brazing material Ztl, that is to say, at about l000 C. when pure silver is used as in the present instance. The pressure may be applied in any suitable manner and may be readily accomplished by assembling the various members as shown in FlG. 1 and applying pressure to members 10 and 11. Sufficient pressure is applied to force out excess solder and insure a thin iilm thereof as well as intimate contact over the interface.

In sealing the peripheries of thin walled member 12 to metallic member lil, l preferably use a brazing material, such as coin silver or copper-silver eutectic alloy, having a melting point lower than that used to effect the seal between thin walled member 12 and ceramic body 11. This avoids the necessity for re-heating the seal at 15 to the melting point of the brazing material used there. On the other hand, where such seals may be made simultaneously, as where the parts are assembled in the relation shown in FIG. l, the same brazing material may be utilized. The formation of such seals is facilitated by a pair of concentric channels 10 formed in the surface of member 10. The solder or brazing material 22 for forming the bonds at 13 and 14- on being fused flows into channels 10. Thin walled member 12 has such dimensions that the peripheries thereof each extend into one of the channels when the parts are brought together (FIGURE l). Thus, the necessity for forming thin walled member 12 to critically exact dimensions is avoided since member 12. may extend more or less into the channels as the case may be.

Ceramic body 11 may be forsterite, 2MgO.SiO2, such as is commercially available under the name Al Si Mag 243 manufactured by American Lava Corporation. The use of this ceramic material has the advantage that the sealing temperature and the duration of heating are substantially less critical than with other materials. Furthermore, the seals may be made in an inert atmosphere, such as dry helium, as well as for example, in a vacuum. While a steatite body has been utilized and bodies of Zircon variety may be feasible, l prefer the particular forsterite body indicated because of the ease with which such seals are made. However, as improved ceramic bodies become available they may be used to advantage.

I may also bond thin walled member 12 to the end 21' of spacer member 21 when the latter is of the same material as ceramic member 11. This is carried out simultaneously with and in the same manner as the seal at 1S. This is done when a seal even stronger than that previously described is desired. When a double bond or seal is thus made, thin Walled member 12 may be made aptwo members.

theY ceramic member 11 must b e such that the joined -portions thereof are free to move relative to each other as a result of unequal expansion and contraction of the Therefore, the spacer member 21 must f 'be moveable relative to either the thin walled member 12 or the metallic member 10. Thus, if the spacer member 21 is sealed to the thin walled member 12, it must not also be sealed to the metallic member 10, so that it can slide therealong as the members and 11 expand and contract unequally.

As pointed out, member 12 is extremely thin. Therefore, the pressure inside and outside of that portion of thin walled member `12 wholly within the envelope should preferably be substantially equalized to avoid damage thereto. This may be readily accomplished by providing one or more small apertures, one of which being indicated at 23 in FIGURE l, through member 12. Such apertures are not through the envelope since the seal at 14 is inside the envelope formed by ceramic member 11, the

-seal at 1S, the outer wall of member 12, the seal at 13 and metallic member 10. The outer wall of member 12,

"i.e., between the seals'at 13 and 15, may, if desired, be

kept away from member 21 by corrugated sheets of molybdenum or other suitable metal.

Referring now to FIGURE 3, electron tube 25 comprises a cathode 26, control electrode 27, and an anode` 28, Cathode 26 may be supported as shown by an annular metallic cathode terminal member 29 which may be copper or other suitable material such as stainless steel coated with silver to improve` the conductivity thereof. Control electrode 27 is similarly supported by annular l grid terminal member 3i). The electrodes are maintained vin insulated spaced relation by means of ceramic tubular members 31, 32 to which terminal members 29, 30 and 28 are sealed. In the present instance anode terminal member 28Vis an integral extension of anode 2S. The

lseal at 33 is of the type described in connection with FIGURES 1 and 2. Thin walled member 34, which is U-shaped in cross section, is sealed at its peripheries tol terminal member 29 and has one surface portion sealed to the end wall of tubular ceramic member 31. Tubular ceramic spacer member 35 rests on terminal member 29 land may be sealed along its opposite end to thin walled of nickel, has both its peripheral portions sealed to the ends of lands 3&5, 39, in thewmanner previously described for the seals at 13 and 14, and is supported on tubular ceramic spacer member 41. One surface of member 40 is sealed to ceramic member 31 or 32 in the manner previously described. The opposite surface of member 40 may be sealed to ceramic member 41, as in the seal at 33.

The anode seal at 37 is illustrative of yet another manner in which a thin walled member may be sealed to an annular terminal member with one periphery extending over the terminal member. For the reasons pointed out in connection with the seal of FIG. 1, the spacer members 35 and 41 must be movable relative to either the terminal members 29 and 3ft or the thin walled members 34 and 40. Thus, if the spacer members are sealed to the thin Ywalled members 34 and 4i?, they must not be sealed to j the terminal members 29 and 30.

When necessary, differences suchv as differences in thermal expansivity between the metal members may be 'compensated for by providing corrugations in the thin walled member as for example in the case of member 40. In FIGURE 4 there is shown still another type of seal.

Here, thin walled member 45 is again U-shaped in crosssection and is sealed along its peripheries to relatively massive metallic member 46. The central portion of member 45 is supported on a tubular spacer member in the form of an annular land 46 formed on member 46. The surface of thin walled member opposite that facing land 46 is sealed as heretofore described to tubular ceramic member 47. Since the ceramic member 47 and thin walled member 45 must be movable relative to the metallic member 46 to accommodate unequal expansion and contraction of the ceramic and metallic members, care must be exercised to avoid the possibility of thin walled member 45 becoming bonded to the surface of land 46 which would result in destruction of the seal. Therefore, when member 45 is nickel and member 46 is stainless steel containing some chromium, the surface of land 46 is treated as by heating in moist hydrogen to form an adherent surface layer of chromium oxide which prevents alloying of the nickel with the steel.

1t is evident from the foregoing that I have provided a novel and highly useful arrangement for effecting seals between ceramicand metal members. Furthermore, seals constructed in accordance with my invention not only are exceedingly strong but also are widely useful. For example, electron tube 25 may be a high frequency, high power electron tube such as maybe used in ultra high frequency television transmission. While the thin walled members form parts of the vacuum envelope and the high frequency lead-ins for the electrodes located within the envelope, forces which result from the higher external pressure tend to reinforce the metal to ceramic seals. Becauseof the thinness and ductility of the thin walled members thermal stresses to which the seals are subjected both during processing of devices such as tube 25 and during their operation do not impair Vthe seals. The thin walled members are suliciently yielding or elastic so as to be constrained or stretched by the contraction and expansion of the ceramic members. Whenthe difference between the coefficients of expansion of the thin walled members and the ceramic members sealed thereto is not as much as that between nickel and forsterite then the thin walled members need not be so yielding.

In any case, it is essential that thin walled members be suflciently thin. In the case of nickel sealed on only one surface to a ceramic member, a thickness of l0 mils K said envelope and connected to said electrode, an annular ceramic member forming part of said envelope and having a substantially planar annular surface, rigid spacing means extending from said terminal member and having an annular substantially planar surface presented toward said ceramic member surface, an annular thin walled metallic member interposed between Vsaid mutually presented surfaces and sealed along one surface intermediate its peripheries to said ceramic member surface, one peripheral portion of said thin walled metallic member forming a part of said envelope and being sealed to said terminal member, another peripheral portion of said thin walled member extending inside said envelope and being sealed to said terminal member, and said surface of said rigid spacing means abutting the surface of said thin YWalled member opposite the surface portion thereof having a substantially planar annular surface and a metallic terminal member each forming part of said envelope,

said terminal member being electrically connected to said electrode, an annular thin walled metallic member having an inner and an outer endless periphery and sealed along one surface intermediate said peripheries to the planar surface of said ceramic member, the peripheral portions of said thin Walled member being sealed to said terminal member in regions spaced inwardly of and outwardly of said ceramic member, and rigid spacing means extending from said terminal member to said thin walled member.

3. An electron tube, comprising an envelope, an electrode within said envelope, a metallic terminal member connected to said electrode and forming a portion of said envelope, a pair of tubular ceramic members disposed in end to end relation, one of said ceramic members abutting said terminal member, the other of said ceramic members forming part of said envelope, an annular thin walled metallic member having inner and outer endless peripheries, said thin walled member being interposed between and abutting said ceramic members and being sealed along one surface thereof intermediate said peripheries to said other ceramic member, and each of the peripheral portions of said thin walled member being sealed to said terminal member in regions spaced from said ceramic members.

4. An electron tube, comprising an envelope, an electrode within said envelope, a metallic terminal member connected to said electrode and forming a portion of said envelope, a pair of tubular ceramic members disposed in end to end relation, one of said ceramic members abutting said terminal member, the other of said ceramic members forming part of said envelope, an annular thin walled metallic member having inner and outer endless peripheries, said thin walled member being interposed between and abutting said ceramic members and being sealed along opposite surfaces thereof intermediate said peripheries to the opposed surfaces of said ceramic members, andv each of the peripheral portions of said thin walled member being sealed to said terminal member in regions spaced from said ceramic members.

5. An electron tube, comprising an envelope, an electrode within said envelope, a tubular ceramic member having a substantially planar annular surface and a metallic terminal member each forming part of said envelope, said terminal member being electrically connected to said electrode, an annular thin walled metallic member forming an endless channel substantially U-shaped in cross section, said thin walled member being sealed along the outer surface of the base portion thereof to the planar surface of said ceramic member, the peripheral portions of said thin walled member being sealed to said terminal member in regions spaced inwardly of and outwardly of said ceramic member, and rigid spacing means extending from said terminal member to said thin walled member and abutting the inner surface of the base portion of said thin walledmember.

6. An electron tube as in claim 5 wherein said rigid spacing means comprises a second tubular ceramic member having one end abutting said terminal member and the opposite end abutting said thin walled member.

7. An electron tube as in claim 6 wherein said second ceramic member has a substantially planar surface abutting said thin walled member and sealed thereto.

8. An electron tube, comprising an envelope, an electrode within said envelope, a metallic terminal member connected to said electrode and forming a portion of said envelope, said terminal member having an annular channel formed therein, an annular thin walled metallic member closing said channel and sealed along its peripheries to said terminal member, annular rigid spacing means in said channel and abutting one surfaceV of said thin walled member intermediate the peripheries thereof, a tubular ceramic member forming part of said envelope and having a substantially planar annular surface sealed to the surface of said thin walled member opposite said one surface thereof in a region spaced from said peripheries.

9. An electron tube, comprising an envelope, an electrode within said envelope, a metallic terminal member connected to said electrode and forming a portion of said envelope, said terminal member having a pair of parallel spaced apart concentric annular lands formed thereon and dening a channel therebetween, an annular thin walled metallic member extending across said channel and sealed along its peripheries to each of said lands, annular rigid spacing means in said channel and abutting in supporting relation a portion of one surface of said thin walled member intermediate the peripheries thereof, a tubular ceramic member forming part of said envelope and having a substantially planar annular surface sealed to that portion of the other surface of said thin walled member opposite to the surface portion abutting said rigid spacing means in a region spaced from said peripheries.

10. An electron tube, comprising an envelope, an electrode within said envelope, a metallic terminal member connected to said electrode and forming a portion of said envelope, said terminal member having an annular land formed thereon having a given width, an annular channeled thin walled metallic member extending over said land and sealed adjacent its peripheries in regions spaced inwardly and outwardly of said land to said terminal member, a tubular ceramic member forming part of said envelope wall and having a substantially planar annular surface sealed to the surface of said thin walled member opposite to but on the side thereof away from said land.

11. An electron tube as in claim lO wherein said land has a coating thereon which inhibits the formation of a bond between said land and said thin walled member.

l2. A laminated structure, comprising a substantially tubular ceramic member having a substantially planar end surface, a metallic member, an annular metallic thin walled member bonding said ceramic member to said metallic member, said thin walled member being adherently joined along peripheral portions thereof to said metallic member and along an intermediate surface portion thereof spaced from said peripheries to said surface of the ceramic member, and rigid spacing means extending from said metallic member to said thin walled member and supporting the portion of said thin walled member joined to said ceramic member.

13. A. laminated structure as in claim 12 wherein said thin walled member is of nickel and has a wall thickness of up to and including .007 of an inch.

14. An electron tube comprising an envelope, an electrode within said envelope, a metallic terminal connected to said electrode and forming part of said envelope, an annular thin walled metallic member sealed at its peripheries to said terminal member, a tubular ceramic member forming part of said envelope and having a substantially flat annular surface sealed to in intermediate portion of said thin Walled member spaced from said sealed peripheries, and rigid means preventing movement of said intermediate portion toward said terminal member while permitting movement thereof away from said terminal member.

15. An electron tube as in claim 14, wherein said rigid means comprises an annular ceramic spacer interposed between and abutting said terminal member and said intermediate portion.

16. An electron tube as in claim l5, wherein said ceramic spacer is sealed to said thin walled member.

17. An electron tube as in claim 14, wherein said rigid means comprises an annular surface on said terminal member abutting said intermediate portion.

18. An electron tube as in claim 17, wherein said last named annular surface is provided by an integral annular land formed on said terminal member, and said thin walled member is a channel of substantially U-shaped cross section extending over said land.

19. An electron discharge device having a radial metallic surface and a cylindrical member of refractoryinsulating material requiring vacuumtight seal therebetween, a flexible metallic bridging element having opposite flanges and with said flanges sealed to and projecting from the said metallic surface, an intermediate portion of said bridging element traversing the end of and sealed to said member, and another element interposed in juxtaposition to the said intermedidate portions of Vsaid bridging element between said flanges and making contact with said metallic surface.

20. An electron discharge device having a radial metallic surface and a cylindrical member of refractory insulating material requiring vacuumtight seal therebetween, a flexible metallic bridging element having opposite flanges and with said flanges sealed to and projecting from the said metallic surface, an intermediate portion of said bridging element traversing the end of and sealed to said member, and another element interposed in juxtaposition to the said intermediate portion of said bridging element and making slidable contact with said metallic surface, said other element and member having `Substantially the same coefllcient of expansion.

2l. An electron discharge device having a radial metallic surface and a cylindrical member of refractory insulating material requiring vacuumtight seal therebetween, a flexible metallic bridging element having opposite flanges and with said flanges sealed to and projectving from the said metallic surface, an intermediate portionV of said bridging element traversing the end of and sealed to said member, and another element interposed vin juxtaposition Vto the said intermediate portion of said bridging element and making contact with said metallic surface, said other element and member having substantially the same coefficient of expansion.

v bridge thereat from said surface, and said member being sealed to said bridge in opposition to said other element.

23. An electron discharge device having a metallic surface transverse to a central axis and acylindrical member of refractory insulating material coaxial thereto, a flexible annular metallic bridge over an annular part of said metallic surface and sealed thereto at the annular peripheries of said bridge, and another element between said bridge and said surface maintaining distance of said bridge thereat from said surface, said other element having slidable engagement with said surface, and said member being sealed to said bridge in opposition to said other element.

24. An electron discharge device having a metallic surface transverse to a central axis and a cylindrical member of refractory insulating material coaxial thereto, a flexible metallic bridge over an annular part of said metallic surface and sealed thereto at the annular pe- Vripheries of said bridge, and an element between said i member having a given transverse dimension, and a metal member having parallel flat surfaces, one end of said ceramic member being hermetically sealed to one of said surfaces of said metal member, said ceramic backing member being sealed to the other of said surfaces of stress said metal member in symmetrical alignment with said tubular ceramic member, said ceramic backing member having a transverse dimension equal to said given transverse dimension of said ceramic member.

26. An electron tube comprising an envelope having a tubular ceramic insulating section and a metal section adjacent said ceramic section, said ceramic section being metalized on its surface adjacent said metal section, a ceramic backing member positioned between said envelope sections, said backing member being metalized on one surface thereof, said backing member being oriented so that it has a ceramic surface in contact with said metal section and said metalized surface thereof faces said ceramic section, an annular metal sealing flange projecting from said metal section, a portion of said flange being sandwiched between said ceramic section and said backing member, and metallic bonds joining the sandwiched portion of said flange to the metalized surfaces of said ceramic section and of said backing member.

27. An electron tube comprising an envelope having a tubular ceramic section and a metal end wall section adjacent one end of said ceramic section, said ceramic section being metalized on its end adjacent said metal-V section, a ceramic backing member positioned between said envelope sections, said backing member being metalized on one surface thereof, said backing member being oriented so that it has a ceramic surface in contact with said metal end wall section and said metalized surface thereof faces said ceramic section, an annular metal sealing flange structure projecting from said metal section across the joint between said metal section and said backing member, a portion of said flange structure being sandwiched between said ceramic section and said backing member, and metallic bonds joining the sandwiched portion of said flange structure to the metalized surfaces of said ceramic section and of said backing member.

28. An electron tube comprising two electrodes separated by a gap, two metal end walls and a ceramic insulating cylinder therebetween, saidend walls being fixed to said electrodes at opposite sides of the gap, two ceramic backing rings axially in line with said ceramicV structures being sandwiched between said ceramic cyl-` inder and the adjacent backing ring, said ceramic insulating cylinder and said ceramic backing rings being metalized on their surfaces facing the sandwiched portions of said flange structures, and fused metallic bonds joining the sandwiched portions of said flange structures to the metalized surfaces of said ceramic cylinder and backing rings.

29.y An electron tube comprising an envelope having a ceramic section and another section adjacent said ceramic section, a ceramic backing member positioned between said envelope sections in sliding abutment with said other section, a metal sealing flange structure projecting from said other section across the joint between said other section and said backing member, a portion of said flange structure being sandwiched between said ceramic section and said backing member, and a metallic bond joining the sandwiched portion of said flange structure to the adjacent surface of said ceramic section.

30. An electron tube comprising an envelope having a ceramic section and anotherrsection adjacent said ceramic section, a ceramic backing member positioned between said envelope sections in sliding abutment with said other section, a metal sealing flange structure projecting from said other section across the joint between said other section and said backing member, a portion of said flange structure being sandwiched between said ceramic section and said backing member, and metallic bonds'joining the sandwiched portion of said Vflange structure to the adjacent'surfaces of said ceramic section and said ceramic backing member.

31. An electron tube comprising an envelope having a ceramic insulation section and a metal section adjacent said ceramic section, a ceramic backing member positioned between said envelope sections in sliding abutment with said metal section, a metal sealing ange projecting from said metal section, a portion of said llange being sandwiched between said insulating section and said backing member, said ceramic insulating section and said ceramic backing member being metalized on their surfaces facing the sandwiched portion of said sealing flange, and fused metallic bonds joining the sandwiched portion of said ange to the metalized surfaces of said insulating section and backing member.

32. An electron tube having an envelope comprising a ceramic tubular section, another tubular section, and an annular metallic sealing structure, said sealing structure including a rst radially extending portion sealed across one end of said ceramic tubular section and a second radially extending portion hermetically joined to one end of said other tubular section, and a ceramic backing ring interposed between said portions in alignment with said ceramic tubular section.

33. An electron tube having an envelope comprising a rst tubular ceramic section, a second tubular ceramic section, and an annular metallic sealing structure, said sealing structure including a first radially extending portion sealed across one end of said rst ceramic section and a second radially extending portion sealed across one end of said second ceramic section, and a ceramic backing ring interposed between said radially extending portions in alignment with said tubular sections and sealed to one of said portions.

34. An electron tube having an envelope containing a plurality of electrodes and comprising a irst tubular ceramic section, a second tubular ceramic section, and an annular metallic sealing structure, said sealing structure comprising a first metallic ring including a radially extending portion sealed across one end of said irst ceramic section and a second metallic ring including a radially extending portion sealed across one end of said second ceramic section, said second metallic ring supporting one of said electrodes within said envelope at the inner periphery thereof and serving as an electrical connection to said electrode, a ceramic backing ring being interposed between said portions in alignment with said ceramic sections and being sealed to one of said portions, said rst and second metallic rings being hermetically joined at their outer peripheries.

35. An electron tube having an envelope comprising a tubular ceramic envelope section, another tubular envelope section, a irst metallic sealing ring, and a second metallic sealing ring, said irst metallic sealing ring being hermetically sealed to one end of said tubular ceramic section, said second metallic sealing ring being hermetically joined to said other envelope section, the outer peripheries of said sealing rings being hermetically joined, and a ceramic backing ring hermetically sealed to said irst metallic ring opposite said tubular ceramic section 36. An electron tube comprising an envelope having a tubular ceramic section and a metal section adjacent said ceramic section, said ceramic section being metalized on its end adjacent said metal section, a first sealing ring comprising a radially extending portion overlapping the metalized end of said ceramic section, a ceramic backing ring coaxial with said ceramic section and positioned between said metal section and overlapping portion of said lirst sealing ring, metallic bonds joining the overlapping portion of said rst sealing ring to the adjacent surface of said backing ring and to the metalized end of said ceramic section, a second sealing ring projecting from said metal section into engagement with said rst sealing ring, and a metallic bond joining said rst and second sealing rings, the coeicient of expansion of said first sealing ring being greater than the coeilicient of expansion of said ceramic section References Cited in the file of this patent UNITED STATES PATENTS 2,472,942 Drieschman et al June 14, 1949 2,647,218 Sorg et al July 28, 1953 2,720,997 Dailey et al Oct. 18, 1955 FOREIGN PATENTS 800,340 France Apr. 27, 1936 

1. AN ELECTRON TUBE, COMPRISING AN ENVELOPE, AN ELECTRODE WITHIN SAID ENVELOPE, A METALLIC TERMINAL MEMBER HAVING PORTIONS THEREOF EXTENDING WITHIN AND OUTSIDE SAID ENVELOPE AND CONNECTED TO SAID ELECTRODE, AN ANNULAR CERAMIC MEMBER FORMING PART OF SAID ENVELOPE AND HAVING A SUBSTANTIALLY PLANAR ANNULAR SURFACE, RIGID SPACING MEANS EXTENDING FROM SAID TERMINAL MEMBER AND HAVING AN ANNULAR SUBSTANTIALLY PLANAR SURFACE PRESENTED TOWARD SAID CERAMIC MEMBER SURFACE, AN ANNULAR THIN WALLED METALLIC MEMBER INTERPOSED BETWEEN SAID MUTALLY PRESENTED SURFACES AND SEALED ALONG ONE SURFACE INTERMEDIATE ITS PERIPHERIES TO SAID CERAMIC MEMBER SURFACE, ONE PERIPHERAL PORITON OF SAID THIN WALLED METALLIC MEMBER FORMING A PART OF SAID ENVELOPE AND BEING SEALED TO SAID TERMINAL MEMBER, ANOTHER PERIPHERAL PORTION OF SAID THIN WALLED MEMBER EXTENDING INSIDE SAID ENVELOPE AND BEING SEALED TO SAIDA TERMINAL MEMBER, AND SAID SURFACE OF SAID RIGID SPACING MEANS ABUTTING THE SURFACE OF SAID THIN WALLED MEMBER OPPOSITE THE SURFACE PORTION THEREOF SEALED TO SAID CERAMIC MEMBER. 